Thread by @thekookreport

🚨A new instalment of #hardcoredd for $ASTS 🚨 Here I summarize the recent FCC filings relating to SpaceX and Starlink to help us understand the problem they have.

This could be one of the all-time unexpected upside surprises for a duopoly: SpaceX was a large overhang for investors and then physics entered the room leaving $ASTS as the only approved player

Here is background on Starlink's D2C program. The key points are they utilize a thin slice of high-band PCS spectrum, which matters in terms of guard band availability, system throughput, and propagation. Starlink repurposed Swarm for the effort, which is key: it was never built for purpose

$ASTS was built from the ground-up to deliver D2C functionality while not creating harmful interference. It does this by creating a nice "square shaped" signal around the utilized frequency and cuts off the "side-lobes" that will splash outside of the intended frequencies and cause interference

As @CatSE___ApeX___ beautifully demonstrated, the large wide beams of Starlink "splash" within the overall area of the satellite's field of view. This *aggregate* interference is the problem. $ASTS has thousands of small, focused beams that can be directed where they need to be and avoid this aggregate interference.

AT&T highlighted the extent of the issue in this filing: fcc.gov/ecfs/document/…

The FCC published initial SCS rules, invited comment, and ultimately passed final rules that dealt with permitted interference levels. Even before those rules were issued, Starlink had been petitioning for higher allowed interference (as measured by Power Flux Density - we will get to this). Starlink has put in filings that it CANNOT COMPLY. Their words.

Let's do some background. Power Flux Density ("PFD") is a measure of how much power from a radio signal is spread over a specific area.

In satellite communications, PFD limits are set to control the amount of signal power that reaches the Earth from a satellite. These limits help prevent the satellite's signals from interfering with terrestrial services (like mobile networks) that operate on adjacent or nearby frequencies.

This is the rule SpaceX cannot meet.

The FCC has referenced PFD with its limit of -120 dBW/m2/MHz. Less negative numbers equate to more power. But...on a log scale. The decibel (dB) is a logarithmic unit used to express ratios, often power ratios in electronics and communications. The logarithmic scale means that each step represents a multiplicative change, not a simple additive change.

When it comes to decibels relative to a watt (dBW), each 10 dB represents a tenfold (10x) difference in power.

However, in the filings we will see Starlink related the PFD to Interference/Noise ratios ("I/N"). They will argue the rules are inequitable because they think they can uphold I/N thresholds so long as they can break the PDF thresholds.

Interference-to-Noise Ratio (I/N) is a ratio that compares the level of interference from an unwanted signal to the noise level in the receiver. It is usually expressed in decibels (dB). A more negative I/N value indicates less interference compared to the noise floor. I/N thresholds are used to ensure that the interference from an adjacent signal does not significantly degrade the performance of a communication system. For example, a typical I/N threshold might be -6 dB, which means the interference power should be six decibels below the noise floor of the receiver.

Here is how the relationship works: if the goal is to keep the I/N ratio at -6 dB, the PFD limit must be set low enough that the signal power reaching the terrestrial network does not cause the interference to exceed this ratio.

If a satellite operator requests a waiver to increase the PFD limit (as SpaceX did, requesting to move from -120 dBW/m²/MHz to -110.6 dBW/m²/MHz), they argue that even with a higher PFD, the interference level will still be manageable (will still meet the desired I/N threshold).

However, others may contest this, saying that increasing PFD could push the I/N ratio above acceptable levels, leading to harmful interference

So why does the FCC use PFD instead of I/N?

The type of the Interference-to-Noise ratio (I/N) for a constant Power Flux Density (PFD) can change based on the type of spectrum. The type of spectrum affects the I/N ratio due to differences in propagation characteristics, usage environments, and receiver sensitivity at different frequency bands.

The choice to regulate PFD directly relates to the need for a more straightforward, enforceable standard that ensures compatibility and reduces interference between different radio services. By setting PFD limits, the FCC directly controls how much power a satellite can transmit towards the Earth’s surface. It’s a clear way to limit potential interference from space-based systems.

Regulating based on I/N would require the FCC to account for the specific technical details of each device, which is impractical given the diversity and continuous evolution of technology.

The problem that Starlink has, which it has admitted to, is that to adhere to the rules it will not have a functional system. SpaceX tells the FCC that by reducing the system power, its system's signal quality would fall apart and users might not be able to connect at all (page 4, para 2 of filing)
fcc.gov/ecfs/document/…

Everything has a trade off and these graphics @CatSE___ApeX___ shared helps visualize what the FCC filings show. The current system interferes. If you turn down the power, the signal quality goes to hell. If you keep the signal quality but use your small sliver of spectrum as a guard band, there is not enough throughput

The result is that Starlink appears to require a new rule. I have no idea what they will do short of a redesign of their system. As @CatSE___ApeX___ put in meme form, Starlink is now going to be trashed by the Swamp monsters because the FCC is not a "move fast and break things" type of agency.

Citations here:

§ Initial AT&T Objection to SpaceX May 23, 2023:
§ SCS Rules:
§ Full SpaceX Docket: ))
§ Verizon Petition to Deny August 12, 2024:
§ AT&T Petition to Deny August 12, 2024:
§ SpaceX Support of Waiver August 22, 2024:
§ T-Mobile Support of Waiver August 22, 2024:
§ FCC Final Rules August 23, 2024: fcc.gov/ecfs/document/…
federalregister.gov/documents/2024…
fcc.gov/ecfs/search/se…
fcc.gov/ecfs/document/…
fcc.gov/ecfs/document/…
fcc.gov/ecfs/document/…
fcc.gov/ecfs/document/…
docs.fcc.gov/public/attachm…

As always, the full DD can be found here.

XOXO, Kook.

https://x.com/thekookreport/status/1797396586907250783